Engines which are utilized in single-engine helicopter applications typically incorporate a freewheeling clutch and a separate torque measuring device which is within or attached to the reduction gear box, or is attached to a drive system of the helicopter in order to establish torque transmission to the main rotor and to monitor the torque value transmitted to the main rotor, or facilitate auto-rotation in the event of a reduction or loss of engine power.
It is known that torque measurement can be achieved by a phase shift measuring device used to measure an angular deviation between two ends of a rotating shaft which transmits torque. One example of such a phase shift measuring device is described in U.S. Pat. No. 4,520,681, issued to Moore et al. on Jun. 4, 1985. The device of Moore et al. includes two slotted disks disposed on a rotating shaft a predetermined distance apart. A slot is formed in the periphery of each of the disks. A light source and a light sensor are mounted stationary with respect to the shaft and proximate to the periphery of each disk. The time duration between signals from the respective sensors is measured and the angular deviation of the two disks can be calculated therefrom. Measurement of this angular deviation determines the amount of twist on the shaft portion between the two disks. The torque can then be calculated from this angle of twist, by a computerized central processing device.
Another example of phase shift measuring devices is described in U.S. Pat. No. 5,918,286, which issued to Smith et al. on Jun. 29, 1999. Smith et al. describe a device for torque measurement of rotating shafts for the purpose of calculating the shaft power without using sensors mounted or glued to the shaft. Accuracy is increased by using only one single optical electronic sensor in the pulse receiver. The light is transmitted from the source/transmitter through optical fiber. The light beam is pulsed within two air gaps by coding disks/gear wheels mounted to the shaft at a convenient distance from each other. The time displacement between the vanes/teeth on the two disks/gear wheels is a measure on the shaft torque, enabling the shaft power to be calculated by a computer. To increase the accuracy in case of shaft vibration, the width modulated pulses are accumulated and averaged at convenient numbers of shaft turns. The accuracy is increased by detecting both rising and falling pulse edges in order to double the number of pulses of each shaft turn.
The angular deviation of the rotating shaft which transmits torque depends on both the torque value being transmitted through the rotating shaft and the resilient property in angle deviation of the rotating shaft, which is inherent in the nature of the shaft material and is further determined by the geometry of the shaft. Thus, the amount of torque transmitted through the rotating shaft can be calculated from the angular deviation of the rotating shaft because the geometry of the rotating shaft and the nature of the shaft material are not variable. Nevertheless, the rotating shaft is generally designed for full load torque transmission and therefore the angular deviation of the rotating shaft at a low torque level will be relatively small, resulting in inaccuracies in the torque measurement. However, such accuracy is needed for example, in helicopter rotor applications.
Therefore, there is a need for a method and apparatus for torque measurement which provide adequate measuring accuracy at various torque levels.